Overview of DeCS's tree structure DeCS follows the tradition of the classification systems and their respective subject heading lists which have been transformed into specialized vocabularies without, however, changing the structures of the classification systems from which they originated. Its tree structure is well-grounded in the division of knowledge in decimal classes and subclasses respecting their conceptual and semantic relationships, and its terms are presented in a hybrid structure of pre- and post-coordination. See also the qualifier hierarchical tree. The distribution of DeCS vocabulary concepts are (2009 version): 25.8% refer to chemical compounds and drugs (category D), both exogenous and endogenous; 20.2% of the total are anatomical terms (category A), organisms (category B) and phenomena and processes (category G); 12.8% of the total refer to diseases (category C); areas such as techniques and equipment (category E), allied sciences (categories F, H, I, J, K, L, M and N) and geographic locations (category Z) altogether represent 21.5%; the area of Public Health (category SP) is represented by 10.3% of the total, Homeopathy (category HP) by 5.8%, Environmental Health (category VS) by 2.5%, and Science and Health by 0.6%. These last four categories were especially developed to better represent the literature generated in the countries of the region.